Question

I have some questions about protocols in OBJ-C.
I often see lines that declare an id object with a protocol, for example:

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{

    id  <NSFetchedResultsSectionInfo> secInfo = [[self.fetchedResultsController sections]objectAtIndex:section];
    NSLog(@"%i",[secInfo numberOfObjects]);
    return [secInfo numberOfObjects];
}

I don’t understand the following line at all:

id  <NSFetchedResultsSectionInfo> secInfo = [[self.fetchedResultsController sections]objectAtIndex:section];

1. What does ” id ” mean?
2. What object does “objectAtIndex:section” return?
3. Why can I call “[secInfo numberOfObjects]” on the object?
   I mean, How’s the method numberOfObjects exists?

I’m trying to lean CoreData but I can’t actually continue without understanding this.
Thank you.

Answer 1

What does id <ProtocolName> mean?

It means “a pointer to any Objective-C object of which the class implements the ProtocolName protocol”.

What object does objectAtIndex:section return?

It returns objects of any class, which conform to the <NSFetchedResultsSectionInfo> protocol.

Why can I call [secInfo numberOfObjects] on the object?

Why not?

I mean, how does the method numberOfObjects exist?

The class of the returned object implements it.